Malic acid, its salts and derivatives are well-known and useful in commerce, inter alia as food acidulants.
Malic acid has been commercially available since the early 1960's; see Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 1981, Vol. 13, pages 103-119 and the art cited therein, all incorporated herein by reference. Kirk-Othmer refers to a Japanese patent which relates to the direct hydration of maleic acid at 180.degree. C. and 1.03-1.21 MPa (150-175 psi). A commercial synthesis of R,S-malic acid is discussed in detail by Kirk-Othmer. This synthesis involves hydration of maleic or fumaric acid at elevated temperature and pressure (180.degree.-220.degree. C./1.4-1.8 MPa, i.e., 200 psi or above) in a titanium reactor under acidic conditions. The reaction is inefficient in that a significant amount of fumarate is generated and has to be separated from the malic acid and recycled. After a twelve-step purification process, malic acid containing less than 1% fumaric acid and less than 0.05% maleic acid is obtained. Thus, the current commercial synthesis suffers at least four serious disadvantages: first, in the inefficiency of the malic acid formation; second, high pressures are needed; third, highly corrosive acid conditions require use of a titanium reaction vessel; and fourth, elaborate and costly purification is required.
Accordingly, it will be appreciated that a simpler means for converting low-cost maleate to malate would be advantageous. The present invention provides such a process.